IGCSE Rate of Reaction讲义
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SECTIONS on this page: 1. What do mean by rate/measurement? * 2. Collision theory of reaction * 3. Factors: 3a concentration, 3b pressure, 3c stirring, 3d particle size/surface area, 3e temperature, 3f catalyst, 3g light * 4. Examples of graphs
1. What do we mean by Rate and how is it measured?
The phrase ‘rate of reaction’ means ‘how fast is the reaction’ or 'the speed of the reaction'. It can be measured as the 'rate of formation of product' (e.g. collecting
gaseous product in a syringe) or the 'rate of removal of reactant'. The speeds of
reactions are very varied.
o Rusting is a ‘slow’ reaction, you hardly see any change looking at it!
o The weathering of rocks is an extremely very slow reaction.
o The fermentation of sugar to alcohol is quite slow but you can see the carbon dioxide bubbles forming in the 'froth' in a laboratory experiment or beer
making in industry!
o    A faster reaction example is magnesium reacting with hydrochloric acid to form magnesium chloride and hydrogen or the even faster reaction
between sodium and water to form sodium hydroxide.
o Combustion when a fuel burns in air or oxygen, is a very fast reaction.
▪  A 'use of words' revision note: Reacting and/or dissolving?
Chemical or physical change?
▪If you take the solids magnesium chloride or sodium
hydroxide and mix them with water they dissolve to form a
solution, but no chemical reaction to form new substances
takes dissolving on its own is basically a
physical change.
▪However, the two substances mentioned above are formed in
a chemical reaction change, where the word 'dissolving' on
react to doits own is inadequate. The phrases reaction with ... or
reaction between ... are much more appropriate, but there is
no denying that the magnesium/sodium dissolve in acid/water,
BUT only because they have formed a water soluble
compound.
o Explosive reactions would be described as ‘very fast’ e.g. the pop of a hydrogen-air mixture on applying a lit splint or the production of a gas to inflate
the air bags safety feature of many cars.
∙The importance of "Rates of Reaction knowledge":
o Time is money in industry, the faster the reaction can be done, the more economic it is.
▪You need to know how long reactions are likely to take.
▪Hence the great importance of transition metals or
enzymes which reduce time and save money.
o Health and Safety Issues:
▪Mixtures of flammable gases in air present an explosion hazard (gas reactions like this are amongst the fastest reactions known).
▪  e.g. Methane gas in mines, petrol vapour etc. are all
potentially dangerous situations so knowledge of
'explosion/ignition threshold concentrations', ignition
temperatures and activation energies are all important
knowledge to help design systems of operation to minimise
risks.
▪Flammable fine dust powders can be easily coal
dust in mines, flour in mills.
▪Fine powders have a large surface area which
greatly increases the reaction rate causing an
explosion. Any spark from friction is enough to initiate
the reaction!
∙  A reaction will continue until one of the reactants is used up.
∙To measure the ‘speed’ or ‘rate’ of a reaction depends on what the reaction is, and can what is formed be measured as the reaction proceeds? Two examples are
outlined below.
∙When a gas is formed from a solid reacting with a solution, it can be collected in a gas syringe.
o The initial gradient of the in cm3/min (speed or rate) gives an accurate measure of how fast a gaseous product is being formed
in  metal/carbonate - acid reaction (forming H2/CO2 respectively). You can
measure the gas formed 30 seconds and plot the graph and
measure the initial gradient cm3/min or cm3/sec.
o The most accurate measurements are made early on in the reaction when the gas volume versus tim
e is almost linear. You can take a series of
measurements and draw the graph (origin 0,0) to get the rate from the
gradient (e.g. cm3/min) or measure the time to make a fixed volume of gas (*
see below).
o If the reaction is allowed to go on, you can measure the final maximum volume of gas and the time at which the reaction stops, though this a very
poor measure of rate, because the reaction just goes slower and slower as the
reactant amounts/concentrations are decreasing - so don't use this as a
method of measuring reaction speed.
o(*) The reciprocal of the reaction time, 1/time, can also be used as a measure of the speed of a reaction. The time can represent how long it takes to form a
fixed amount of gas first few minutes of a metal/carbonate - acid reaction, or
the time it takes for so much sulphur to form to obscure the X in the sodium
thiosulphate - hydrochloric acid reaction. The time can be in minutes or
seconds, as long as you stick to the same unit for a set of a set of
experiments varying the concentration of one of the reactants.
Examples of reactions involving gas formation
o(i) metals dissolving in acid ==> hydrogen gas, (test is lit splint => pop!),
▪  e.g. magnesium + sulphuric acid ==> magnesium sulphate +
hydrogen
▪Mg(s) + H2SO4(aq)==> MgSO4(aq) + H2(g)
o(ii) carbonates dissolving in acids => carbon dioxide gas, (test is limewater => cloudy),
▪calcium carbonate (marble chips)  + hydrochloric acid ==> calcium
chloride + water + carbon dioxide
▪CaCO3(s) + 2HCl(aq)==> CaCl2(aq) + H2O(l) + CO2(g) o and (iii) the manganese(IV) oxide catalysed decomposition of hydrogen peroxide (è oxygen gas, test is glowing splint => relights)
▪hydrogen peroxide ==> water + oxygen
▪2H2O2(aq)==> 2H2O(l) + O2(g)
▪can all be followed with the gas syringe method.
o You can do all sorts of investigations to look at the effects of
▪(a) the solution concentration,
▪(b) the temperature of the reactants,
▪(c) the size of the solid particles (surface area effect),
▪(d) the effectiveness of a catalyst on hydrogen peroxide
decomposition.
The shape of the graph is quite characteristic (see diagram above and notes below).
o The reaction is fastest at the start when the reactants are at a maximum (steepest gradient in cm3/min).
o The gradient becomes progressively less as reactants are used up and the reaction slows down.
o Finally the graph levels out when one of the reactants is used up and the reaction stops.
o The amount of product depends on the amount of reactants used.
o The initial rate of reaction is obtained by measuring the gradient at the start of the reaction. A tangent line is drawn through the first part of the
graph, which is usually reasonably linear from the x,y origin 0,0.
▪This gives you an initial rate of reaction in cm3 gas/minute,
▪Typical results from a gas producing reaction are shown below, for
different amounts or concentrations of reactants. How to calculate
the reaction rate is explained below.
▪  e.g. for run q [ ], after 2 mins, 20 cm3 of gas formed, so the rate of
reaction is 20/2 = 10 cm3/min.
▪From the graph of results you can measure the relative rate of
reaction from (i) the initial gradient in cm3/min (see on diagram above),
(ii) you can estimate from the graph the volume of gas formed after a
particular 3 minutes or (iii) you can estimate the time it takes
to form a particular volume of gas. (i) is the best the best
straight line covering several results at the start of the reaction.
▪
▪Keeping the temperature constant is really important for a 'fair test' if you are investigating speed of reaction/rate of reaction factors such
as concentration of a soluble reactant or the particle size/surface area
of a solid reactant. On the advanced gas calculations page,
temperature sources of error and their correction are discussed in
calculation example Q4b.3, although the calculation is above GCSE
level, the ideas on sources of errors are legitimate for GCSE level.
▪Note that if the temperature of a rates experiment was too low
compared to all the other experiments, the 'double error'
would occur again, but this time the measured gas volume
and the calculated speed/rate of reaction would be lower than
expected.
∙The rate of a reaction that produces a gas can also be measured by following the mass loss as the gas is formed and escapes from the reaction flask.
o The method is ok for reactions producing carbon dioxide or oxygen,
o but not very accurate for reactions giving hydrogen (too low a mass loss for accuracy).
o The reaction rate is expressed as the rate of loss in mass from the flask
g/min based on the initial gradient (see graph below).
∙When sodium thiosulphate reacts with an acid, a yellow precipitate of sulphur is formed and forms the basis of a good project for assessment.
o To follow this reaction in your investigation you can measure how long it takes for a certain amount of sulphur to form.
o You do this by observing the reaction down through a conical flask, viewing a black cross on white paper (see diagram below).
o The X is eventually obscured by the sulphur precipitate and the time noted.
o sodium thiosulfate + hydrochloric acid ==> sodium chloride + sulfur dioxide + water + sulfur
o Na2S2O3(aq) + 2HCl(aq)==> 2NaCl(aq) + SO2(aq) + H2O(l) + S(s)

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